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Abstract

Background

Since sugarcane areas have increased rapidly in Brazil, the contribution of the sugarcane
production, and, especially, of the sugarcane harvest system to the greenhouse gas
emissions of the country is an issue of national concern. Here we analyze some data
characterizing various activities of two sugarcane mills during the harvest period
of 2006-2007 and quantify the carbon footprint of sugar production.

Results

According to our calculations, 241 kg of carbon dioxide equivalent were released to
the atmosphere per a ton of sugar produced (2406 kg of carbon dioxide equivalent per
a hectare of the cropped area, and 26.5 kg of carbon dioxide equivalent per a ton
of sugarcane processed). The major part of the total emission (44%) resulted from
residues burning; about 20% resulted from the use of synthetic fertilizers, and about
18% from fossil fuel combustion.

Conclusions

The results of this study suggest that the most important reduction in greenhouse
gas emissions from sugarcane areas could be achieved by switching to a green harvest
system, that is, to harvesting without burning.

Background

Increasing atmospheric greenhouse gases (GHG) and its relation to human activities
have pressured the productive sector to mitigate its GHG emission [1]. Developing country-specific emission factors and activity data have been a tough
challenge particularly for non-Annex I countries which are recognized mostly as certain
groups of developing countries that are vulnerable to the adverse impacts of climate
change. Therefore the demand for assistance for non-Annex I countries to improve their
inventories is likely to rise and should be effectively made [2]. Among the main practices that have caused national concern in Brazil, the harvest
system is highlighted, especially in sugarcane agricultural areas, which in most regions
are still based on residues burning. In contrast, the so-called green harvest, without
burn, keeps large amounts of crop residues in soil surface [3].

Sugarcane residues represents 11% of the worldwide agricultural residues [4], and while sugarcane areas have increased rapidly in Brazil, few papers quantify
its impact on air quality due to the land use, especially considering the burning
practice [5-7]
. Brazil is the biggest worldwide sugarcane grower with a 622 millions ton production
(2008/2009) concentrated in 7.8 millions of hectares [8]. Those are mostly driven to ethanol (55%) and sugar (45%) derivatives, while sugarcane
industrial process generate also 11.3 TWh of electric energy produced in the power
plants in most of the sugarcane mills, corresponding to 3% of all electric energy
consumed in the country [8]. Sugarcane is one of the world's major food-producing crops providing about 75% of
the sugar for human consumption [9]. Projections indicate the biomass importance in near future that will provide up
to 20% of all worldwide energy used in the end of 21 century [10]. Adding efforts to reduce emission from energy and deforestation sectors, it is also
a top priority to implement innovative programs that promote mitigation in the agricultural
and livestock sectors [11].

The goal of this work was to determine a scope for sugarcane mills emissions within
its own boundary and quantify the GHG emissions sources related to the sugarcane production
in agricultural sector in Brazil. It was applied the Intergovernmental Panel on Climate
Change (IPCC) methodology [12], chapter 11, N2O emissions from managed soils, and CO2 emissions from lime and urea application, chapter 2 Generic methodologies applicable
to multiple land-use categories and The First Brazilian Inventory to Mobile Combustion
[13]. It was considered the total sugar production in order to determine the carbon footprint
in terms of carbon dioxide equivalent (CO2eq) released to the atmosphere per area, ton of cultivated sugarcane and sugar produced.

Results and Discussion

Figure 1 presents the partition of GHG emission for each emission source considered in this
study. Based on the scenario and studied year, total company's GHG emission was 164,878
ton of CO2eq corresponding to 2.41 ton of CO2eq emitted for each cropped hectare. Some authors showed emission of 3.24 ton of CO2eq ha-1 considering 60% of area harvested with burning practice and emission related to fertilizers,
herbicides and pesticides manufacturing phase incorporated in this amount [14] while in our scope it was considered emissions related to company's boundary emissions,
only. Others authors consider also emissions from the manufacture and distribution
of agricultural inputs used for Brazilian sugarcane production presenting a net contribution
of CO2from the sugarcane agro industry to the atmosphere as 3.12 ton per ha [15]. On the other hand, results have shown an average from 0.32 ton C ha-1yr-1 accumulated in the first 20 cm depth to 1.95 ton C ha-1yr-1 for the top 40 cm layer referring to green harvest adoption instead of burning, corresponding
to as much as 7.15 ton CO2eq ha-1 yr-1. This could be effectively considered a CO2 sequestration from atmosphere due the conversion of burned to green harvest [11], which despite the uncertain, has the potential to mitigate all GHG emission of this
sector.

Residues burning accounted for 72,462 ton CO2eq, around 44% of total emission, equivalent to 1.21 ton of CO2eq for each burnt hectare, being 72% of this associated to CH4 emission only. In our inventory CO2 and CO emissions were not included as net GHG emission to atmosphere when the crop
residue burning is considered. Some authors also do not compute those gases as net
emission when referred to the burning practice [12,16]. CO2 sunk by sugarcane crops in following year compensates the amount of CO2 and CO (that once in atmosphere rapidly transforms in CO2) emitted by burning. Computing the total CO2 captured by photosynthesis relative to the 2006/2007 crop season with area of 68,541
ha, there is something around 5,133,212 ton of CO2, equivalent to 74.9 ton of CO2 ha-1sunk by sugarcane crops from atmosphere. This value is comparable to the one presented
for sugarcane crops, with an amount of 107.2 ton of CO2 ha-1 year-1[17].

Direct and indirect N2O emission due to the synthetic fertilizers use, organic composts and harvest residues
caused an emission of 49,827 ton of CO2eq, corresponding to 30% of the total emission. Fossil fuel combustion (diesel use)
and lime application contributed with 30,252 and 12,338 ton of CO2eq, respectively, mostly due to CO2 only. Substitution from diesel to biodiesel has been cited as an alternative to reduce
net CO2 emission in this sector [17]. Also, CO2 emission due to diesel use could be reduced from 15 to 29% by alternative tillage
systems i.e. reduced tillage, as a consequence of fuel savings [18].

Figure 2 presents the partition of direct and indirect N2O emissions in terms of their sources. Organic fertilizers applied on soil resulted
in 7,678 ton of CO2eq, corresponding to 15% of total N2O emitted in this sector. Synthetic fertilizers application resulted in 33,181 ton
of CO2eq (67%) and it considers only the use emission, not the ones associated to the fertilizer
production. The application of chemical or organic fertilizers on soil can stimulate
N2O and NO production via nitrification (aerobic) and denitrification (anaerobic) biochemical
processes [19,20]. The input of organic fertilizers to agricultural soils is considered an important
source of N2O [21] with both chemical and organic fertilizer applications being the major sources of
NH3 [9,22,23]
. In our inventory these were some of the mainly sources of GHG emission to atmosphere,
believing that such aspect is representative of sugarcane production areas.

Residues from sugarcane remained on field resulted in 8,968 ton of CO2eq, coming from residual N content which is converted to N2O through nitrification, aerobic microbial oxidation of ammonium to nitrate and denitrification
process which is the anaerobic microbial reduction of nitrate to nitrogen gas (N2). Nitrous oxide is a gaseous intermediate in the reaction sequence of denitrification
and a by-product of nitrification that is ultimately released into the atmosphere
[12]. The application of nitrification inhibitors has been suggested as an option for
decreasing N-fertilizer use and consequently such emission [24]. Strategies that increase N-fertilizer efficiency, reducing N2O emission have also been suggested by several authors [24-26]
.

Table 1. presents estimations of GHG emission per kilogram of sugar produced, per hectare
and per ton of sugarcane produced. According to this study each ton of sugarcane processed
released 26.5 kg CO2eq to atmosphere, resulting 241 kg of CO2eq for each ton of sugar produced. Emission value for sugar beet production (Life
Cycle Assessment - LCA) suggests an emission of 900 kg of CO2eq per ton of sugar produced [27]. LCA should be a suitable tool to assess the environmental impact associated with
agricultural production [27], but this provides different methodologies to compare GHG emission in agricultural
sector. In Brazil, some authors presented amounts of 222 kg CO2eq ton-1 of sugar in the so-called organic production, without burn and without synthetic fertilizers
N application [28]. That study considered emissions related to sugar transport, energy imbed in the
equipments and agricultural machines and also emissions related to production of chemical
supplies, resulting amounts of 34.08 kg CO2eq per ton of sugarcane processed, a reduction of 32% in GHG emission when compared
to conventional practices that resulted in 50.44 kg CO2eq, considering the same scope [29].

Conclusions

Considering the studied scenario, with 87% of the total area managed with burning
practice and 13% of green harvest, GHG emission ratio was 241 kg CO2eq ton-1 of produced sugar. Each hectare of sugarcane cropped transferred to the atmosphere
2,406 kg of CO2eq per year. This indicate that a more sustainable agricultural production systems
as conservation tillage and direct planting during the re-planting season, as well
as rationalizing the N fertilizers use might be achieved to reduce GHG emissions in
sugarcane areas. The total sugarcane production of 6,221,025 ton resulted in an emission
ratio of 26.5 kg of CO2eq per ton of sugarcane processed. Considering only emissions from application and
not emission from production of synthetic fertilizers N applied to soils, each kilogram
used transfers to the atmosphere 6.45 kg CO2. Sugarcane field burning practice impacted on 1.21 ton of CO2eq per hectare burnt, considering only GHG net emissions. Responsible for 44% of total
GHG emission, the conversion of sugarcane burning system to green harvest could reduce
emissions in this sector. Considering actual production process, the company emission
baseline to 2006/2007 season was 164,878 ton of CO2eq. The mitigation of GHG emissions from sugarcane areas could be achieved either
by reducing burning and fertilization practices or using soil as a carbon sink. Applications
of standardized scope, emission factors and emissions boundaries within company's
activities only, show be necessary to promote comparison among companies and GHG emission
reduction.

Methods

To elaborate this work it was adopted the reference data of 2006/2007 informed by
appropriated company sector, harvest period (from May 2006 to April 2007) from a sugarcane
mill located in the southern Brazil, northeast region of São Paulo State, Brazil.
The total sugarcane cropped area of the studied sugarcane plants in the period was
68,541 hectares (ha), resulting in a sugarcane and sugar production of 6,221,025 and
684,850 ton, respectively for both mills. In this scope we did not consider emissions
related to the production of any supply (synthetic fertilizers, cement, herbicides,
pesticides, steel, etc.) considering it to each company the decision to provide its
own inventory.

Estimates of how much C was stored by crops in one year was calculated by considering
the total sugarcane dry mass content as 53%, being 25% stalks, 12% trash, 4% green
leaves and 12% roots [30]. Mill database informed an average sugarcane yield of 90.76 ton ha-1. To convert carbon (C) to carbon dioxide (CO2) it was applied the 44/12 factor (1 kilogram of carbon correspond to 3.67 kg CO2 captured), considering the C content in sugarcane dry matter as 42.46% [31].

The net emission was related to residues burning in the field, methane (CH4) and nitrous oxide (N2O), [12], direct and indirect N2O emissions from managed soils [12] and CO2 emissions referred to lime application. Emissions of CO2, carbon monoxide (CO), CH4, N2O, and NMVOC (non-methane volatile organic compounds) referred to the use of fossil
fuel (total diesel consumption for all equipments and agricultural machines involved
within production) were considered [13] according to Mobile Sources Brazilian National Inventory. All values were converted
to CO2 equivalent (CO2eq) following the individual global warming potential for a period of 100 years for
each gas, using 1 to CO2 [12], 3 to CO [32], 21 to CH4, 310 to N2O [12] and 3.4 to NMVOC (only to mobile combustion) [12]. Table 2. summarizes the scope considered in this work with partition in sector and emission
sources.

Agricultural residues burning

The impact of residues burning in GHG emission took into account data from sugarcane
crop varieties grown and harvested in the burnt areas only (59,820 ha). Total sugarcane
yield was 5,643,786 ton in burned areas, corresponding to an average yield of 94.4
ton ha-1. Average values of residue per yield ratio were accounted in 19% of the varieties
cropped in the burned areas indicating a residue per yield ratio of 0.205, resulting
in an average of residue mass available to combustion of 19.3 ton per hectare. According
to an extended review [33], the value of residues yield from different plant varieties in São Paulo state is
around 19.1 ton ha-1. This is also similar to the amount found by other authors [34,35], of 18.2 ton of sugarcane residues per hectare. The combustion factor applied in
this work was 0.80 [12].

The sugarcane residues burning result is not only CO2 emissions but also other GHG or precursors, including carbon monoxide (CO), methane
(CH4), non-methane volatile organic compounds (NMVOC) and nitrogen (N2O, NOx) species [36]. Usually in the cropland and grassland areas only non-CO2 emissions are considered, due to the assumption that those would be counterbalanced
by CO2 removals from the subsequent re-growth of the vegetation within one year [1]. The same applies to CO, as this is converted in CO2 rapidly once in atmosphere [1]. NOx emission was not considered as a net GHG because its global warming potential is very
uncertain [1].

Different emission factors related to sugarcane residues burning have been registered
in literature [37]. In this work it was used the ones suggested by IPCC, [12], Chapter 2, Generic Methodologies Applicable to Multiple Land-Use Categories (Equation
1). Those were 2.7 and 0.07 to CH4 and N2O (all values in g kg-1 dry matter burnt) respectively [38].

Direct and indirect emissions of nitrous oxide from managed soils

In this analysis, the emission sources considered were nitrogen from synthetic fertilizers
and organic composts applied on soils (filter cake and vinasse), in addition to the
harvest residues (Equation 2 - Direct emissions and Equation 3 and 4 - indirect emissions).
In order to account for the total amount of N synthetic fertilizer applied we adopted
a standard nitrogen demand from sugarcane agricultural areas in our region [39], which is around 75 kg of nitrogen (N) ha-1 year-1. On the other hand, the amount of filter cake and vinasse applied in the production
areas was informed by the company as 119,140,000 kg and 1,872,338,000 liters respectively.
The N content used was 1.4 and 1.1%, for filter cake and vinasse, respectively, and
those values were informed by the company, after the characterization. The N content
in the filter cake was based on 25% of its dry mass, while N content of vinasse was
considered as being 0.368 kg N m-3 applied [40].

The amount of N in harvest residue was inferred according to current methodology [12] considering sugarcane average yield for harvested without burn areas as 66.18 ton
ha-1. As ratio residue/yield ratio is close to 0.205, 13.75 ton ha-1 of above ground residues, having 1.27% of N content on it, was available for combustion
[15].

Once the amount of N in each of those composts was know it is possible to infer the
N2O emission due to the direct application of fertilizers, taking into account the emission
factor given by IPCC (2006). This calculation simply converts 1% of the total N input
to N2O emission [12].

Indirect emissions of N2O involves two different pathways, the first one is the volatilization of N as ammonium
(NH3) and oxides of N (NOx), and the following deposit of these gases and their products NH4+ and NO3- in soil surface or lakes [12]. The nitrification and denitrification processes on soils transform some of these
products to N2O returning back to atmosphere. According to the followed methodologies [12], 10% of N input of synthetic fertilizers and 20% of N input of the organic compost
is volatilized and transformed into N2O, after nitrification and denitrification process on soils. Nevertheless, 1% of N
applied on soils is transformed into N2O, resulting in an indirect emission effect. Leaching and runoff are also secondary
pathways that could result in N2O emissions, in some regions. It is assumed that 30% of total N applied as synthetic
and organic fertilizer and unburned residues is leached or runoff but this can also
return as N2O by an emission factor of 0.0075, (or 0.75%)[12].

CO2 emissions due lime application

The lime used during 2006/2007 season was the dolomite one CaMg(CO3)2, totalizing 25,883 ton applied in 11,423 ha (2.27 ton ha-1). For those it was considered an emission factor of 0.13 ton of CO2 per ton of dolomite lime applied [12].

Emissions from mobile combustion

In this scope only motors powered by diesel were took into account for emission due
to fossil fuel combustion (Equation 5), including company proper machinery, the transport
of sugarcane stalks to the mills and all supplies within the company boundary and
labor transport, totalizing 7,058,709 liters. For the third part transport (sugarcane
stalks, supplies and labors) it was considered only the annual consumption of diesel
(2,526,761 l) totalizing 9,585,470 liters of diesel used to calculation.

Data of diesel fleet was obtained by the company mechanization sector according to
a very careful control of vehicles and its fuel consumption and traveled distance
per year (kilometers year-1), being 25.77% of trucks and buses, 52.22% of agricultural machinery and 22.01% of
cars powered by ethanol. The data from sugar transport after company's boundary were
not considered. The total fleet of cars used in the production cycle is powered by
the same ethanol produced by the mill, hence it's assumed that the ethanol GHG emissions
(CO2) is reabsorbed in the next crop cycle and not accounted. The mobile sources were
classified considering vehicles per category and manufacturing year, motor power and
traveled distance per vehicle during the study (2006/2007 season).

Estimations of the GHG emission related to fossil fuel use in this study considered
direct and indirect emissions of CO2, CO, CH4, N2O and NMVOC, according to the Brazilian Inventory recommendations [41]. Emission factors applied were also established (Air Control Program by Auto Motors
Vehicles Pollution)/CETESB [25] in association with IBAMA (Brazilian Institute of Environment), considering type
of fuel and vehicles. The methodology takes into account four steps: first it is considered
data from fleet per vehicle category and second, the use of diesel, distributed by
categories, and distance traveled. The next steps were to establish the emission factors,
considering each vehicle, each vehicle's GHG emission per gas, and the conversion
to CO2eq using an Excel worksheet to arrange and calculate all results and determine total
emissions per vehicle and the total GHG amount. To determine the diesel emission factors
it was used diesel density as 852 g liter-1 and specific consume of 195 g kWh-1, data from Brazilian fuel. The emission factors (g liter-1) used in this report was established in 06 phases according to vehicles manufacturing
year.

(5)

Eg,t = emission of gas g by fleet year/model t.

EFg,t = emission factor of gas gfrom vehicle's year t; (g L-1)

FC,t = Fuel consumption per vehicle's year t (liters).

Competing interests

The authors declare that they have no competing interests.

Authors' contributions

All authors participated in detailed discussions that led to this review paper. EBF
conceived the document design and coordination, calculated the results and drafted
the manuscript. ARP originally contributed to data analyses, interpretation, drafting
and editing the manuscript. RR and NLSJ provided intellectual input on available data
and previous analyses, and on the synthesis, presentation and interpretation needed
for this review. All authors read and approved the final manuscript.

Acknowledgements

We are grateful to Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP),
Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) and Coordenação
de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) for support.

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